1. Field of the Invention
The present invention relates to a dimensionally stable laminate which is formed from at least two layers of web and at least one layer of a laid structure and is useful in particular for reinforcing bitumen sheets.
2. Description of the Prior Art
GB-B-1 517 595 discloses a similar material in which a laid network of glass fiber is incorporated into an organic fiber nonwoven and the resulting assembly is then consolidated by application of an acrylate binder.
EP-B-110 039 similarly discloses an at least 3-layered laminate composed of web and laid components. In one embodiment, this known laminate comprises two layers of organic fiber web enclosing a layer of laid mineral fiber, for example glass fiber. In this known material, the bonding between the web layers and the laid layer is by hot melt adhesive.
German utility model DE-U-7 424 706 discloses a filter material comprising a web onto which a woven or laid structure is needled in such a way as to form a fiber pile on the nonwoven remote side of the woven or laid structure.
The materials disclosed in the cited references GB-B-1 517 595 and EP-B-110 039 are recommended for manufacturing bituminous roofing sheets.
Yet, when used in this way, they have the serious defect of tending to warp and crack under thermal stress. This defect can lead to production problems even at the stage of the known laminates being impregnated with hot bitumen, but, when the bituminous sheets are hot laid on the roof or later exposed to fluctuating insolation, it can also lead to leakiness in the roof membrane produced.
These problems are addressed in DE-A-3 941 189 and it is stated there that these defects of existing materials are due to the very different reactions (e.g. extensibility, modulus changes, shrinkage) of the fiber materials used in the web and in the reinforcement (polyester fiber in the web and glass fiber in the reinforcement) to mechanical and/or thermal stress. It is therefore proposed in said reference that to remove the stated defects of existing materials nonwovens be reinforced not with textile sheet materials, such as glass mats or weaves, but with individual reinforcing strands incorporated in the nonwoven in a spaced-apart parallel arrangement. Bonding is by chemical binding agents, by needling and/or thermally.
The materials obtained according to this proposal do indeed show improved thermal stability. However, they do not have the particularly important high mechanical stability required for reinforcing bituminous sheets, and the manufacture of such nonwovens, reinforced with individual filaments, proved to be very costly and troublesome.
It is an object of the present invention to provide laminates which are dimensionally stable under thermo-mechanical stress and are easy to manufacture.